Cooling water corrosion is a multifaceted damage mechanism that occurs in systems using water to remove heat from process
equipment, such as heat exchangers, cooling
towers, and piping. The corrosion is driven by: 1. Dissolved Salts: Primarily chlorides, but also sulfates, nitrates, and carbonates, which increase electrolyte conductivity and
promote electrochemical corrosion. 2. Gases: Dissolved oxygen (O₂) and carbon dioxide (CO₂) act as cathodic reactants or form acidic species, accelerating corrosion. 3.
Organic Compounds: Hydrocarbons or organic contaminants from process leaks can alter water chemistry, promote fouling, or support microbial growth. 4. Microbiological Activity:
Microbiologically influenced corrosion (MIC) from bacteria like sulfate-reducing bacteria (SRB) creates localized corrosive environments.
Mechanisms of Corrosion: The damage manifests as general corrosion (uniform metal loss) or localized corrosion (pitting, crevice corrosion, stress corrosion cracking (SCC), or
MIC), depending on the material, water chemistry, and operating conditions.
Electrochemical Corrosion: Cooling water acts as an electrolyte, facilitating electron transfer between anodic (metal dissolution) and cathodic (e.g., O₂ reduction) sites:
Fe Fe2++2e (anodic) and O2+2H2O+4e 4OH (cathodic) also the Chlorides and other salts enhance conductivity, accelerating corrosion. Oxygen Corrosion: Dissolved O₂
drives general and pitting corrosion, forming tubercles (iron oxide deposits) that promote under-deposit corrosion. MIC: Bacteria (e.g., SRB) produce corrosive metabolites (e.g.,
H₂S), causing pitting or channeling under biofilms. Chloride-Induced Corrosion: In stainless steels, chlorides initiate pitting and crevice corrosion by penetrating the passive
oxide layer, followed by an autocatalytic process (HCl formation,
DAMAGE pH drop).
MECHANISMS SCC: Chlorides
AFFECTING FIXED Eor ammonia (in copper alloys) cause stress corrosion cracking
QUIPMENT IN THE REFINING INDUSTRY 101in susceptible materials
under tensile stress. Fouling: Deposits (mineral scales, silt, organic matter, corrosion products) create occluded areas, promoting under-deposit corrosion and MIC.
What Makes Cooling Water Corrosion Worse?
3.20 Cooling Water Corrosion Corrosion, Fouling, and MIC – A Triplet to Manage Together
These three mechanisms are interrelated. For instance, dissolved O₂ in cooling water (one of many dissolved elements)
drives general and pitting corrosion, forming tubercles (iron oxide deposits- a corrosion product which is one of many
3.20.1 Description of Damage kinds of fouling) that promote under-deposit corrosion, and fouling creates dead zones where MIC can thrive, and MIC
can further initiate corrosion.
General or localized corrosion of carbon steels and other metals caused by dissolved salts, gases, organic
compounds, or microbiological activity in cooling water systems.
3.20.2 Affected Materials
Carbon steel, all grades of stainless steel, copper and copper alloys, aluminum and aluminum alloys, titanium,
and nickel alloys.
3.20.3 Critical Factors
a) Cooling water corrosion, fouling, and MIC are closely related and should be considered together. Fluid
temperature, type of water (fresh, brackish, or salt water) and the type of cooling system (once-through, open
circulating, or closed circulating), oxygen content, and fluid velocities are critical factors.
In fresh water systems, process-side temperatures >60°C in carbon steel exchangers
b) Higher cooling water temperature causes increased corrosion rates.result in scaling and under-deposit corrosion.
In seawater heat exchangers, outlet temperatures >45°C cause CaCO₃ scaling,
leading to pitting in 316 SS tubes.
Increasing cooling water heat exchanger outlet temperatures and or process-side inlet temperatures tend
to increase exchanger bundle corrosion rates as well as fouling tendency. If the process-side
temperature is above 140 °F (60 °C), a scaling potential exists with fresh water and becomes more likely
as process temperatures increase and as cooling water inlet temperatures rise. Brackish and salt water
outlet temperatures above about 115 °F (45 °C) may cause serious scaling.This happens because warmer water holds less oxygen and
encourages salt precipitation and bioactivity.
c) Increasing oxygen content tends to increase carbon steel corrosion rates.Oxygen is a key oxidizer in the corrosion electrochemical reaction.
d) Fouling may be caused by mineral deposits (hardness), silt, suspended organic materials, corrosion
products, mill scale, and marine and microbiological growth. Fouling creates occluded areas where chlorides concentrate, lowering pH and accelerating pitting.
Biofilms trap corrosive species, promoting MIC and under-deposit corrosion.
< 1m/s
e) Low velocities can promote increased corrosion. Velocities should be high enough to minimize fouling and
dropout of deposits but not so high as to cause erosion. Velocity limits depend on the pipe diameter or heat
exchanger tube material and water quality.
Oversized channel heads, water side exchanger shells, or dead-legs can be areas of low or stagnant
flow. Large volumes reduce velocity, causing sedimentation. Also, Stagnant piping sections trap deposits and microbes.
Generally, velocities below about 3 fps (1 m/s) are likely to result in fouling, sedimentation, and increased
corrosion in fresh and brackish water systems. Accelerated corrosion can also result from dead spots or
stagnant areas if cooling water is used on the shell side of condensers/coolers rather than the preferred
tube side.
> 4 m/s
f) High velocities can also lead to accelerated corrosion. Cause abrasion or erosion-corrosion by removing protective oxide layers or corrosion products.
Exchanger tubes can see a dramatic increase in flow velocity. High velocities at tube inlets (e.g., due to nozzle design) cause localized metal loss.
g) 300 series SS, depending on the specific alloy and the water and metal temperatures, can suffer pitting and
crevice corrosion. At temperatures above about 140 °F (60 °C), 300 series SS can also suffer Cl− SCC even
in freshwater systems where a chloride salt concentrating mechanism is in place. This is a particular concern
with the tubes in water-cooled heat exchangers, i.e. condensers and coolers. (See 3.17.)
h) Brass (Cu-Zn) alloys can suffer dezincification in fresh, brackish, and salt water systems. They can also suffer
SCC if any ammonia or ammonium compounds are present in the water or on the process side if cross-
leakage occurs.
i) ERW carbon steel pipe or exchanger tubes may suffer severe weld and/or HAZ corrosion in fresh or brackish
water. Electric resistance welded (ERW) pipes have welds and heat-affected zones (HAZ) with altered microstructures (e.g., martensite), making them anodic relative to
the base metal. In fresh/brackish water, welds/HAZ corrode preferentially, forming grooves along fusion lines.
When titanium is galvanically coupled to a more anodic material (e.g., carbon steel), titanium acts as the cathode, absorbing hydrogen:
2H++2e H2(on titanium)
At >75°C,102
hydrogen diffuses into the titanium lattice, forming brittle hydrides,
API RECOMMENDED causing
PRACTICE 571 embrittlement and cracking mainly in seawater
exchangers
j) When connected to a more anodic material, titanium may suffer severe hydriding embrittlement. Generally,
the problem occurs at temperatures above 165 °F (75 °C). (See 3.66.)
3.20.4 Affected Units or Equipment
Cooling water corrosion is a concern with cooling towers, piping, pumps, water-cooled heat exchangers, and any
other equipment associated with cooling water systems.
3.20.5 Appearance or Morphology of Damage
a) Cooling water corrosion can result in many different forms of damage including general corrosion, pitting
corrosion (Figure 3-20-1), MIC, SCC, and fouling.
b) General or uniform corrosion of carbon steel occurs when dissolved oxygen is present. Many oxidizing
biocides also increase this tendency.
c) Localized corrosion may result from under-deposit corrosion, crevice corrosion, or MIC.
d) Wavy or smooth corrosion at nozzle inlets or outlets and exchanger tube inlets may be due to flow
accelerated corrosion, erosion, or abrasion.
e) Corrosion at ERW weld areas will appear as grooving along the weld fusion lines.
3.20.6 Prevention/Mitigation
a) Cooling water corrosion (and fouling) is best managed by proper design, operation, and chemical treatment
of cooling water systems.
b) Process-side inlet temperatures of water-cooled exchangers should be maintained below 140 °F (60 °C).
c) Minimum and maximum water velocities must be maintained, particularly in saltwater systems.
d) The metallurgy of heat exchanger components may need to be upgraded for improved resistance, especially
in waters with high chloride content, low velocity, and/or poorly maintained water chemistry, where exchanger
process-side temperatures are high, or where there is simply the desire to extend tube life.
e) Periodic mechanical cleaning of tube IDs and ODs should be performed in order to maintain clean heat
transfer surfaces.
Tube-side flow ensures uniform velocity, reducing fouling and corrosion compared to shell-side flow, which creates dead spots.
f) With very few exceptions, cooling water should be on the tube side to minimize stagnant areas.
g) Installation of sacrificial anodes on the cooling water side of water-cooled heat exchangers can increase the
life of channel heads, tubesheets, and tubes to a certain extent, as long as they are galvanically coupled to
the anodes.
3.20.7 Inspection and Monitoring
a) Cooling water should be monitored for process conditions that affect corrosion and fouling, including but not
limited to:
pH,
oxygen content,
cycles of concentration,
biocide and other chemical residual,
DAMAGE MECHANISMS AFFECTING FIXED EQUIPMENT IN THE REFINING INDUSTRY 103
biological activity,
iron and manganese count,
cooling water outlet temperatures,
hydrocarbon contamination, and
process leaks.
b) Periodic calculation of overall heat exchanger performance (U-factors) will provide information on potential
scaling and fouling. These could be an indication that corrosion damage is occurring in the piping, exchanger
tubes, and/or other equipment in the system.
c) Strategically placing continuous corrosion monitoring devices on the system, such as corrosion coupons, ER
probes, or online monitoring sensors, can provide an early indication of increased corrosion rates that need
further evaluation.
d) Areas with sharp reduction or large increases in diameter should be considered for velocity survey locations
as velocities that are either too high or too low can dramatically affect the damage rate of the equipment.
Several types of flow meters are available that can be used to check the velocity of water in the cooling water
system.
e) When water sides of exchangers are opened for inspection, checking the sacrificial anodes (when installed)
may indicate the relative corrosivity of the cooling water and if the sacrificial anode has been consumed and
needs replacement.
f) Exchanger tube inspection can be used to establish corrosion rates and predict tube life in order to plan for
tube or tube bundle repair/replacement. Some nondestructive methods to inspect tubes are as follows.
RFT is commonly used for inspection of ferrous (carbon steel) tubes. RFT has an equal sensitivity to ID
and OD indications and can detect and size corrosion and pitting as well as baffle cuts.
RFT: Remote Field Testing
IRIS: Internal Rotary
Inspection System ECT is the preferred method for non-ferromagnetic materials as it has a higher probability of detecting
all types of damage than ultrasonic methods. High sensitivity to pitting, cracking, and wall loss.
Example: ECT detects pitting in 316 SS tubes in seawater exchangers.
IRIS is used when a higher flaw detection and sizing capability is needed (compared to the other
methods), but it is slower, and thorough tube cleaning is required prior to inspection. IRIS can be used
on both ferrous and non-ferrous materials. IRIS is most commonly used on carbon steel tubes.
g) A destructive method for evaluation is extracting and splitting representative tubes to gain access to the
internal surfaces for direct examination. This method is most often used on failed tubes that require
replacement and is useful in determining the cause of the tube failure. It may also be used on tubes where
significant damage has been indicated and needs verification. The knowledge gained from this method may
aid in tube material selection and can help create mitigation plans to avoid future damage.
Extract suspect tubes (e.g., from failed or heavily pitted areas).
Split longitudinally to inspect ID surfaces visually and microscopically.
3.20.8 Related Mechanisms Analyze corrosion products, pit morphology, and crack patterns.
Microbiologically induced corrosion (3.45), Cl− SCC (3.17), galvanic corrosion (3.31), concentration cell corrosion
(3.19), and brine corrosion (3.10).
3.20.9 References
1. T.J. Tvedt, Jr., “Cooling Water Systems,” Corrosion Control in the Refining Industry, NACE Course Book,
NACE International, Houston, TX, 1999.
2. H.M. Herro and R.D. Port, The Nalco Guide to Cooling Water System Failure Analysis, McGraw-Hill, New
York, NY, 1991, pp. 259–263.
COOLING WATER CORROSION
Description Appearance
Scaling and corrosion inside Wide variety of appearances,
LCS systems caused by scaling, pitting, general LOWT
dissolved O2/CO2 gases and
salts
s
m
Inspection:
I "IBM
nTj
Opening up flanges for VT of pipe/vessel
internal surfaces.
Chemical monitoring of process fluid for
corrosive conditions.
Critical factors: Scaling/corrosion increases > 60°C (140°F).
Low fluid velocity promotes corrosion.
Dissolved 02 and C02 increase corrosion rates
dramatically.
Corrosion occurs rapidly under scaling.
FFP/Severity: Generally causes leak-before-break, particularly
in lower temperature systems.
Local-v-general LOWT is unpredictable - highly
dependent on pipework geometry and flow
patterns.
Stainless steels can suffer stress corrosion
cracking (SCC) - more likely to result in
unpredictable cracking failure.
References: API 571 (4.3.4)
104 API RECOMMENDED PRACTICE 571
3. NACE SP0189, Online Monitoring of Cooling Water Systems, NACE International, Houston, TX.
4. NACE SP0300, Corrosion of Metals and Alloys—Corrosion and Fouling in Industrial Cooling Water
Systems—Part 1: Guidelines for Conducting Pilot-scale Evaluation of Corrosion and Fouling Control
Additives for Open Recirculating Cooling Water Systems, NACE International, Houston, TX.
5. NACE/EFC Joint Publication, Monitoring and Adjustment of Cooling Water Treatment Operating Parameters,
NACE International, Houston, TX.
Figure 3-20-1—Cooling water corrosion on the ID of a carbon steel
heat exchanger tube operating at 85 °F (30 °C).
